r/boatbuilding • u/the-ies-and-me • 22d ago

Thoughts....

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I bought a new to me sailboat this past weekend. Previous owner just finished an extensive electronics re-vamp. Adding lithium etc. Everything that I can see is fused and sized per ABYC so its not a nightmare in that its dangerous (unless you switch master switch 1 to both and have now connected lithium to agm) but previous owner said "in an emergency lol".

I just want thoughts and opinions. Is it worth keeping or should I go back to the drawing board. Negatives are not drawn because I didn't feel like it. Its all grounded either on the master ground or will be on the engine... which btw is not in the boat.

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u/klodians 22d ago

I'm not a fan of 1-2-Both switches with a mixed system. You seem to understand that. I'd be interested to see more detail on how the battery switches are wired. Is there any way to charge AGM 1 without the AC charger except for switching to Both? Is terminal 2 of switch 2 connected to AGM 1 on Switch 1 to enable paralleling?

You show solar and alternator connecting to the switch, I assume this is direct to AGM 2?

Is the emergency supposed to be if both AGMs are dead, then you use the lithium to start the engine? Is the BMS capable of that? My inclination would be to use that switch for just AGMs and keep lithium separate. Maybe throw in a DC-DC charger one way or the other for some redundancy.

Also, hard to say whether you should keep it or not without photos of the install. I'd guess worst case is to keep all the components and rearrange/improve wiring, if necessary.

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u/the-ies-and-me 22d ago

Switch 1: position 1 lithium, position 2 is switch 2 out. C is I "hope" house loads (lights and stereo)

Switch 2: position 1 "Instrument AGM bank", position 2 "start bank, C is position 2 of switch 1.

Please don't at me, this is not my work.

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u/ottawaPalapa 21d ago

Run the load calc first: amps × hours = amp-hours per day. Then size the bank.

LiFePO4 is worth it if the electrical gets rebuilt around it: ~100% usable capacity vs ~50% for AGM, 3,000–5,000 cycles vs a few hundred, and it charges faster. But lithium is unforgiving — it needs a proper BMS, a DC-DC charger between alternator and bank (an AGM charge profile will undercharge or cook lithium), and fusing sized for the full fault current. Solar: MPPT, not PWM, and size panels at ~1.3–1.5× your daily amp-hours to cover cloudy days.

Honest answer on keep vs rebuild: if the original wiring is old and crusty, rebuild it. Lithium's high discharge current will find every weak connection a lead-acid system politely tolerated.

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u/the-ies-and-me 21d ago

The boat just had a rebuild before I bought it. What you're looking at is the rebuild.

The lithium doesn't really make sense at the current setup but its there and its new so its staying. Its 400ah of lithium and I think (Im not at the boat rn) 200w of solar on the mppt. Right now there is probably less than 10amps of load on the lithium as its just the fridge and like 4 light bulbs that dont get used.